MOS type transistors are a fundamental building block within integrated circuits. Consequently, there is a persistent push to make such devices smaller, faster, etc. The switching speed of a transistor is obviously an important characteristic since it dictates, at least in one respect, how fast the circuits which employ such devices operate. Presently, the switching speed of a transistor is not limited by the channel transit time (i.e., the time required for charge to be transported across the channel); instead, the switching speed is limited by the time required to charge and discharge the capacitances that exist between the device electrodes and between the interconnecting conductive lines and the substrate.
One way of appreciating the transistor capacitances is through an exemplary cross section, as illustrated in prior art FIG. 1. The transistor, designated at reference numeral 10, includes a p-type region 12 (sometimes referred to as the body), such as a P-well in a CMOS type process. The body 12 has an n-type drain region 14 formed therein and a lightly doped drain extension region 16. Likewise, a source region 18 and a lightly doped source extension region 20 is formed in the body 12. As is well known in the art, the extension regions 16 and 20 are used to help overcome short channel transistor effects as device dimensions continue to shrink. A doped polysilicon gate 22 overlies a thin gate oxide 24 which defines a channel region 26 therebeneath in the body 12.
An effective circuit diagram illustrating the various transistor capacitances is illustrated in prior art FIG. 2. As seen in prior art FIG. 2, capacitances exist between the various device electrodes and between the electrodes and the body region. The drain-to-body capacitance (C.sub.db) and the source-to-body capacitance (C.sub.sb) are illustrated in both prior art FIGS. 1 and 2 and are referred to often as junction capacitances. The value of the junction capacitances are a function of both the cross sectional area of the junctions as well as the doping concentrations of the regions, respectively.
One attempt to increase the performance of the transistor 10 of prior art FIG. 1 reduces the junction capacitances by forming the transistor on an insulating region. Such a transistor device structure is called a silicon-on-insulator (SOI) device and is illustrated in prior art FIG. 3. The SOI transistor, designated at reference numeral 30, has components similar to the transistor 10 of prior art FIG. 1. In the SOI transistor 30, however, the body 12 is not formed in the bulk semiconductor material 12 as in FIG. 1, but rather overlies an insulating layer 32 such as silicon dioxide (SiO.sub.2). The insulating layer 32, in turn, overlies a bulk semiconductor material 34.
The SOI transistor 30 provides several performance advantages over traditional bulk transistor devices. Initially, since each device can be completely isolated from one another (as opposed to sharing a common body), better individual device isolation is achieved, which prevents circuit latch-up conditions. In addition, since at least a portion of the drain region 14 and the source region 18 abut the insulating layer 34, the cross sectional area of the source/body and drain/body interfaces is reduced and thus the junction capacitance is significantly reduced.
Although SOI devices provide several advantages over prior art bulk type devices, SOI transistor also have several disadvantages. One disadvantage of SOI transistors could be (depending upon the application) the lack of bulk silicon or body contact to the transistor. In some cases it is desirable to connect the SOI body region 12 to a fixed potential in order to avoid "floating body effects." Use of a body contact for each transistor device, however, undesirably increases the device size and thus is not an amenable solution.
The floating body effects refer generally to various hysteresis effects which are associated with the body 12 being allowed to float relative to ground. Two such floating body effects include the "kink" effect and the parasitic lateral bipolar effect. The "kink" effect originates from impact ionization. When the SOI transistor 30 is operated at a relatively high drain-to-source voltage, channel electrons having sufficient kinetic energy cause an ionizing collision with the lattice, resulting in carrier multiplication near the drain end of the channel. The generated holes build up in the body 12 of the device 30, thereby raising the body potential. The increased body potential reduces the threshold voltage of the transistor 30, thus increasing the transistor current, which results in a "kink" in the transistor current/voltage (I/V) curves.
The second floating body effect includes the parasitic lateral bipolar effect. As discussed above, if impact ionization generates a large number of holes, the body bias may be raised to a sufficient voltage so that the source/body p-n junction becomes forward biased. When this junction becomes forward biased, minority carriers are emitted into the body 12 which causes a parasitic lateral npn bipolar transistor to turn on. Such parasitic transistor action leads to a loss of gate control of the transistor current and is therefore highly undesirable.
Therefore there is a need in the art for a devices and methods of manufacture for providing transistor devices having lower junction capacitance without altering the fundamentals of the device operation.